Bridges have been around ever since humans began to move themselves -- and their goods -- from one place to another. Early bridge engineers had to do little more than fell a tree across a ravine or stream, but they soon discovered that they could span greater distances and haul heavier loads by putting more time and energy into their structures.

Roman engineers perfected the stone arch and used it to build aqueducts -- bridges that conveyed fresh water great distances -- and similar structures to carry traffic across streams and rivers. But the Industrial Revolution took bridge building to new heights and even greater lengths.

By the end of the 19th century, record-breaking bridges seemed to come fast and furious. The Brooklyn Bridge, completed in 1883, was the longest spanning bridge in the world -- until Scottish engineers completed the Firth of Forth Bridge just seven years later.

A new century brought longer, more amazing bridges. In 1937, the Golden Gate Bridge stretched its 8,981-foot (2,737-meter) back over the treacherous waters of the San Francisco Bay.

Today, engineers continue to test the limits of science and imagination. They experiment with innovative materials, designs and methods of construction. Their finished products move cars and trains. They also move people's souls. What follows is a survey of some of the most breathtaking bridges of the modern era. We've limited ourselves to just 10 bridges, but not to any particular region of the world.

The Chinese are not new to bridge building. The Anlan Bridge, a suspension bridge first constructed in A.D. 300 using bamboo cables, still carries pedestrians across the 1,000-foot (305-meter) Min River [source: NOVA]. More than 1,700 years later, in the Jiangsu Province, Chinese engineers completed a project that includes a 4,888-foot (1,490-meter) suspension bridge and a 2,575-foot (758-meter) cable-stayed bridge. Known as the Runyang Bridge, it was the country's longest bridge -- for a while.

Today, the Hangzhou Bridge, which opened in May 2008 after nine years of planning and construction, holds the honor as China's longest bridge. It stretches a mind-boggling 22 miles (36 kilometers) across the Qiantang River at the Yangtze River Delta on the East China Sea. That's almost 25 times longer than the Runyang Bridge and nine times longer than Japan's pride and joy, the Akashi Kaikyo Bridge! The cable-stayed bridge snakes across the open ocean, carrying six lanes of traffic in both directions. Commuters have to pay 80 yuan, or almost $12, to cross it, but the toll fee is nothing compared to the bridge's price tag of 11.8 billion yuan ($1.72 billion). If it lasts as long as expected -- 100 years -- it should have a long time to pay for itself.

Up next, we're headed west across Asia to the very eastern edge of Europe. There we'll find the Strait of Bosporus and the next amazing bridge on our list.

The Golden Era of Bridges

Modern bridges, such as the Hangzhou Bay Bridge, owe much to structures that preceded them. Bridge building took a great leap forward in the 19th century as engineers in Europe and the United States applied the principles of physics to erect some of the best-known bridges the world has ever seen. The Brooklyn Bridge has been a landmark of New York City since it opened in 1883. Likewise, the Tower Bridge, which began carrying traffic over the Thames River in 1894, is practically synonymous with London. Interestingly, some people didn't care for the visual aesthetic of the Tower Bridge, calling it "monstrous" and "preposterous" [source: PBS].

9

The Bosporus Bridge

The Bosphorus Bridge at the Nov. 5, 1973, opening ceremony by President Koroturk on the 50th anniversary of the founding of the Turkish Republic.

The Strait of Bosporus, also known as the Istanbul Strait, connects the Black Sea to the Sea of Marmara. It also separates Asia from Europe. As such, it holds great strategic value during times of war and peace. Although attempts to bridge the narrow body of water date back to the Persian Empire, the most impressive structure didn't open for business until 1973. That's when engineers completed the Bosporus Bridge, a 4,954-foot- (1.51-kilometer-) long suspension bridge that remained the only bridge in the world linking two continents until the Fatih Sultan Mehmed Bridge suspended its roadway above the strait nearly 15 years later.

Even with a companion bridge stealing some of its glory, the Bosporus Bridge remains a grand sight, especially at night. Thanks to an LED lighting system, the bridge's towers and zigzagged cables glow brilliantly in different colors.

Day or night, the Bosporus Bridge shuttles traffic between continents on an eight-lane deck. Each direction has three lanes for vehicles, plus one emergency lane and one sidewalk. Pedestrians were allowed to access the bridge until 1977, when authorities stopped the practice for safety reasons. They temporarily suspended the rule in May 2005 when Venus Williams and Ïpek Senoglu played a tennis match on the bridge to promote the inaugural Istanbul Cup in Turkey. Fans gathered on two of the bridge's lanes to watch the players trade volleys from Asia to Europe and back again.

The next bridge on our list has never hosted a tennis match, but it has become one of England's hottest tourist attractions.

8

Gateshead Millennium Bridge

Millennium Bridge at dusk, viewed from Gateshead over the River Tyne to Newcastle upon Tyne

In a basculebridge, one or two pieces of the bridge deck, known as leaves, swing up to provide clearance for boat traffic passing underneath. They open quickly and efficiently but aren't the most elegant pieces of architecture. This was the challenge facing engineers when officials in Gateshead, England, announced a contest in 1996 to design an innovative cycle and pedestrian bridge to span the Tyne River. The new structure had to allow ships to pass underneath without blocking the view of other nearby bridges or interfering with cultural activities taking place on either bank.

The winning design solved the problem, not with a bascule-type system, but with a never-seen-before tilting mechanism. Here's how it works. The bridge is made up of a pair of steel arches. In its "down" position, one arch forms the deck of the pedestrian and cycle path. The other arch sits at a 90-degree angle to the first, with cables strung between the two to provide support for the deck. When the bridge needs to move to its "up" position, eight electric motors tilt both arches as a single, rigid structure. As one arch lowers, the other rises to act as a counterbalance.

When the 413-foot- (126-meter-) long bridge opened in 2000, 36,000 people lined the banks of the river to watch the bridge's inaugural tilt [source: Gateshead Millennium Web site]. And in 2007, its likeness appeared on a new pound coin from the Royal Mint.

Our next entry -- the tallest vehicular bridge in the world -- also wowed people when it opened in 2004.

7

Millau Viaduct

Stage 18 of the 92nd Tour de France heads under the Millau Viaduct, the world's tallest vehicular bridge.

The Eiffel Tower, which looms over the Paris skyline, measures 1,063 feet (324 meters) high from its four-footed base to the tip of its flagpole. Now imagine tearing the tower from its moorings and carrying it southeastward, through the Massif Central region of France to the Cévennes Mountains. Now imagine setting the tower down in the Tarn Valley, a deep gorge surrounded by high, rolling hills near the town of Millau, France. Now, finally, picture lining up six more similar towers and stringing a roadway across the valley. Et voila -- the Millau Viaduct, a massive bridge that opened in 2004 to carry the A75 motorway south to Beziers.

In reality, the highest point of this cable-stayed bridge is taller than the Eiffel Tower by 62 feet (19 meters) and only a little bit shorter than the Empire State Building (1,250 feet or 381 meters tall, if you're wondering). A car traveling over the span is actually suspended 890 feet (271 meters) above the valley. Drivers who overcome their fear of heights can shave 62 miles (100 kilometers) and four hours off the long trip from Clermont-Ferrand to popular destinations along the Mediterranean Sea. The bridge also reduces pollution by relieving congestion that used to occur as cars queued up to cross a much older and smaller bridge in Millau.

Another amazing cable-stayed bridge is the Octavio Frias de Oliveira Bridge in Brazil. That's the next stop on our tour.

6

Octavio Frias de Oliveira Bridge

São Paulo, Brazil, just down the coast from Rio de Janeiro, is known for its extravagance. It's a cosmopolitan city, with a flourishing nightlife and a lively arts scene. It's also Brazil's financial hub -- home to some of the wealthiest people in the country. Because of these qualities, it's not surprising that the paulistanos (São Paulo inhabitants) designed and built one of the most unique bridges in the world.

Named after the Brazilian media mogul Octavio Frias de Oliveira, the cable-stayed bridge spans the Pinheiros River, which bisects the western side of the city. The bridge is only 2,953 feet (900 meters) long and not particularly high at 453 feet (138 meters), but its design features two curved decks that cross each other through an X-shaped supporting tower. The two roadways, suspended from the tower by 144 cables, link the São Paulo districts of Brooklin and Real Parque.

Motorists crossing the bridge at night enjoy a spectacular show courtesy of LED lights. The lighting system enables bridge authorities to illuminate the X-shaped tower with different colors for commemorative dates or for special events. But it's not all about special effects. A series of 140-watt bulbs shine on the roads to ensure the safety of drivers.

Innovative safety features also define the next bridge we're going to cover. To get to it, we need to travel out of South America, through Central America and into the United States.

Sky Walking

Pedestrian bridges may not be as long as vehicle or train bridges, but they can still be impressive. The Campo Volantin Footbridge in Bilbao, Spain, uses a graceful arch to suspend a glass deck above the Bilbao River. In Malaysia, the Langkawi Sky-Bridge, less than 6 feet (1.8 meters) wide, allows visitors to enjoy a 360-degree view from the top of Mount Mat Cincang. And in Vancouver, British Columbia, you can walk through the treetops of a lush rainforest as you cross the 450-foot- (147-meter-) long Capilano Suspension Bridge.

5

St. Anthony Falls Bridge

On Aug. 1, 2007, a steel deck truss bridge carrying motorists in and out of Minneapolis collapsed into the Mississippi River during the evening rush hour. An investigation revealed that 16 gusset plates, which connected steel beams in the center span to the load-bearing columns, were too thin to function properly. As a result, they fractured and caused the bridge to collapse. Within hours, officials vowed to rebuild the bridge and to restore public confidence by incorporating a variety of advanced safety features. The result was the St. Anthony Falls Bridge, which opened in September 2008.

This box girder bridge truly is a marvel of modern engineering. It's made with high-performance concrete, which provides great strength and inhibits corrosion. More than 15 million pounds (6.8 million kilograms) of rebar -- all of it coated with epoxy to prevent the steel from getting weak and brittle -- reinforce the concrete [source: McCarthy]. At the same time, 323 sensors embedded in the bridge substrate constantly monitor the integrity of the structure, collecting data that can be analyzed to determine unusual stress points or other trouble spots. While it forms a solid foundation, the concrete is also cleaning the air. That's because it's made with cement containing TX Active, an agent that, in the presence of light, breaks down air pollutants, including carbon monoxide, nitrogen oxide and benzene.

High-tech materials such as TX Active are becoming commonplace in modern bridges. One such bridge is located to the east of Minneapolis, in Fond du Lac, Wis.

London Bridge Is Falling Down

Bridge collapses like the one in Minnesota in 2007 are rare, but they do happen. On May 24, 1847, a post-and-beam railway bridge over the River Dee in Wales collapsed, killing five people. Seventy people were killed when the Tay Bridge, which spanned the Tay River in Scotland, came down in gale-force winds on Dec. 28, 1879. And the Tacoma Narrows Bridge collapsed just four months after it opened on July 1, 1940, from high winds. Luckily, the collapse didn't kill anyone.

4

DeNeveu Creek Bridge

Concrete, a hard and porous composite of cement, sand, aggregate and gravel, forms the foundation of most bridges. It holds up well to compressive forces (those that push down at right angles), but not as well to tensile forces (those that act along the length of the material to pull it apart). Engineers use steel bars, also known as rebar, to reinforce the tensile strength of concrete.

Unfortunately, steel rebar can corrode when exposed to freshwater or saltwater, which can cause distress in the concrete. One method to prevent this wear and tear involves coating the rebar with epoxy to shield the steel from corrosive chemicals. But scientists at the University of Wisconsin-Madison have developed another solution -- a reinforcing matrix made from a novel fiber-reinforced polymer. Because it is nonmetallic, the polymer material won't corrode, which means the concrete remains stronger longer.

The first real-world test of this new and improved concrete is the DeNeveu Creek Bridge, a 40-meter (131-foot) span along Highway 151 in Fond du Lac, Wis. Driving over the bridge, you would never know there is anything special hidden inside the nondescript box girder bridge. But the real payoff will come far into the future. The bridge deck is durable enough to last at least 75 years, whereas most traditional bridge decks last between 30 and 40 years before they need to be replaced. That means the DeNeveu Creek Bridge, completed in 2005, may not need any significant maintenance until 2080!

Next, we're headed back to Europe and a border-crossing bridge that links Denmark and Sweden.

3

Oresund Bridge

The Oresund Bridge may have been difficult to build, but the result of all that hard work was breathtakingly beautiful.

Building a single-purpose bridge is a complex engineering feat. Now add other complicating factors, like a bridge to carry both trains and cars. And join that bridge seamlessly with an immersed tunnel. These were the design requirements facing engineers when they began planning how to connect Denmark and Sweden across the Oresund Strait. Their solution was one of the largest infrastructure projects in European history: the $3 billion Oresund Fixed Link [source: Lundhus].

The Fixed Link actually consists of three major parts. If you're traveling from Copenhagen to Malmö, Sweden, the first thing you encounter is an immersed tunnel 2.5 miles (4 kilometers) long. Back at sea level, you emerge from the tunnel onto Peberholm, a Danish artificial island built from material dredged up during construction. After Peberholm, you access the final leg of the Fixed Link, a cable-stayed bridge stretching for 5 miles (8 kilometers) and ending on Swedish soil.

Collectively, the Oresund Fixed Link is a miracle of modern engineering. It is the world's longest cable-stayed bridge for both road and railway [source: Oresund Bridge Web site]. Its towers are 669 feet (204 meters) high and provide a navigational clearance of 187 feet (57 meters) under the main span. The bridge features two levels, with the railway running along the lower deck and the roadway on the upper.

Luckily, Oresund Bridge builders didn't have to deal with earthquakes and other seismic activity during construction. The engineers in charge of the next bridge on our list weren't so lucky.

2

The Akashi Kaikyo Bridge

The Akashi Kaikyo Bridge may lose its title as the world's longest suspension bridge. A proposed bridge over the Strait of Messina that would link Sicily to mainland Italy could steal its title.

Japan is both a bridge-building dream and a nightmare. Japan consists of four main islands and 4,000 smaller islands -- a veritable mother lode of options and opportunities for bridge crossings. Unfortunately, Japan is also located in the Ring of Fire, an area where large numbers of earthquakes and volcanic eruptions occur. That makes building big structures a supreme engineering challenge. With the Akashi Kaikyo Bridge, Japanese engineers were up to the challenge.

The Akashi Kaikyo Bridge opened in 1998 and stunned the world with its epic size. It spans 2.4 miles (3.9 kilometers) across the Akashi Strait, with a central section stretching almost half of that length. Its towers soar 928 feet (283 meters) above the water, and its cables carry tensile forces of 132,000 tons (120,000 metric tons). It's not just the crowning glory of Japan's elaborate bridge system; it's the longest spanning suspension bridge, with the highest towers, in the world. What makes the Akashi Kaikyo Bridge even more amazing is the fact that it weathered a 7.2-magnitude earthquake during construction. The temblor struck on Jan. 17, 1995, creating a new fault near the bridge. This shifted the bridge's foundations and expanded the central span of the bridge by 2.6 feet (80 centimeters) and one side span by almost a foot (30 centimeters). Luckily, the earthquake didn't damage the towers. Engineers increased the length of the cables, redesigned the girders to accommodate the increased length of the bridge and proceeded with no further mishaps.

Back in North America, another long bridge tested the skill and ingenuity of engineers.

1

Chesapeake Bay Bridge-Tunnel

Oneil Lewis, collects a toll from a motorist at the Chesapeake Bay Bridge Tunnel in Virginia Beach, Va. The structure celebrated its 40th anniversary in 2004. It is considered the world's largest bridge-tunnel complex.

Two bridges span the Chesapeake Bay, a long estuary sandwiched between Virginia and Maryland. The first is the Chesapeake Bay Bridge, a dual-span, 4.32-mile- (6.95-kilometer-) long bridge that connects the eastern and western shores of Maryland. Although this bridge offers an interesting collection of designs in a single structure -- cantilever, arch and suspension -- it pales a bit when it is compared to its cousin located a few miles to the south. The official name of this second bridge is the Lucius J. Kellam, Jr. Bridge-Tunnel, more commonly known as the Chesapeake Bay Bridge-Tunnel.

As its name suggests, the link consists of a series of bridges and tunnels to span the 17 miles (27.4 kilometers) of brackish water separating Cape Charles and Virginia Beach, Virginia. The majority of the bridge-tunnel complex is above the water, supported by more than 5,000 piers. Most of these piers are relatively short, so that the bridge seems to skim just above the surface of the water. To allow ships to pass, two 1-mile- (1.6-kilometer-) long tunnels carry traffic beneath the bay's primary navigation channels. Man-made islands, each approximately 5 acres (2 hectares) in size, are located at each end of the two tunnels and act as transition points between the tunnels and bridges.

When the original bridge-tunnel complex (what is now the northbound side) opened in 1965, it won the American Society of Civil Engineers award for "Outstanding Engineering Achievement." It was also designated "One of Seven Engineering Wonders of the Modern World" in 1965. Thirty years later, engineers began construction on a second link (the southbound side), which opened to traffic in 1999.

Still hungry for more information about bridges and how they work? Explore the links on the next page.